Conventional photographic materials based on silver halide are used for a large variety of applications. For instance, in the prepress sector of graphic arts rather sensitive camera materials are used for obtaining screened images. Scan films are used for producing colour separations from multicolour originals. Phototype setting materials record the information fed to phototype and image setters. Relative insensitive photographic materials serve as duplicating materials usually in a contact exposure process. Other fields include materials for medical recording, duplicating and hard copy, X-ray materials for non-destructive testing, black-and-white and colour materials for amateur and professional still photography and materials for cinematographic recording and printing.
Silver halide materials have the advantage of high potential intrinsic sensitivity and excellent image quality. On the other hand they show the drawback of requiring several wet processing steps employing chemical ingredients which are suspect from an ecological point of view. As a consequence it is undesirable that depleted processing solutions would be discharged into the public sewerage; they have to be collected and destroyed by combustion, a cumbersome and expensive process.
In the past several proposals have been made for obtaining an image that can be formed using only dry development steps without the need of processing liquids as it is the case with silver halide photographic materials.
As a particular alternative for conventional silver halide chemistry dry imaging elements are known that can be image-wise exposed using an image-wise distribution of heat. When this heat pattern is indirectly generated by the conversion of radiation, e.g. laser radiation, into heat these types of dry imaging elements are called heat mode materials. When the heat pattern is provided directly, e.g. by means of a thermal head, these elements are called thermal recording materials or thermographic materials. Both types of elements offer the advantage in addition to an ecological advantage that they do not need to be handled in a dark room nor any other protection from ambient light is needed. Heat mode recording materials, based on change of adhesion, are disclosed in e.g. U.S. Pat. Nos. 4,123,309, 4,123,578, 4,157,412, 4,547,456 and PCT applications WO 88/04237 and WO 93/03928.
In still another type of heat mode recording materials information is recorded by creating differences in reflection and/or transmission in the recording layer. The recording layer has high optical density. The conversion of radiation into heat brings about a local temperature rise, causing a change such as evaporation or ablation to take place in the recording layer. As a result, the irradiated parts of the recording layer are totally or partially removed, and a difference in optical density is formed between the irradiated parts and the unirradiated parts (cf. U.S. Pat. Nos. 4,216,501, 4,233,626, 4,188,214 and 4,291,119 and British Pat. No. 2,026,346). In a preferred embodiment the recording layer of such heat mode recording materials is made of a metal, e.g. bismuth.
Still another type of non-conventional materials as alternative for silver halide is based on photopolymerisation. The use of photopolymerizable compositions for the production of images by information-wise exposure thereof to actinic radiation is known since quite a while. All these methods are based on the principle of introducing a differentiation in properties between the exposed and non-exposed parts of the photopolymerizable composition e.g. a difference in solubility, adhesion, conductivity, refractive index, tackiness, permeability, diffusibility of incorporated substances e.g. dyes etc. The thus produced differences may be subsequently employed in a dry treatment step to produce a visible image and/or master for printing e.g. a lithographic or electrostatic printing master.
Another dry imaging system working according to photo mode and known since quite a while is 3M's Dry Silver technology. It is a catalytic process which couples the light-capturing capability of silver halide to the image-forming capability of organic silver salts.
An image forming system which is chemically very similar to Dry Silver but works according to heat mode since a photosensitive silver, halide is absent is disclosed in European patent application Appl. No. 94200794, filed 24 Mar. 1994. Here a method is disclosed for the formation of a heat mode image comprising the steps of:
(1) preparing a donor element by coating on a support one or more donor layers containing, distributed over said one or more layers, a reducing agent, a radiation to heat converting compound, and optionally a polymeric binder; PA1 (2) preparing an acceptor element by coating on a support an acceptor layer containing a reducible organic silver salt and a polymeric binder; PA1 (3) bringing said donor layer and said acceptor layer in close contact with each other; PA1 (4) information-wise exposing the contacting elements with laser radiation, thus inducing the partial or complete transfer of said donor layer(s) to the acceptor element and/or diffusion of said reducing agent into the acceptor element; PA1 (5) peeling apart the donor and acceptor elements PA1 each of Y.sup.1. Y.sup.2. Y.sup.3 and Y.sup.4 (same or different) represents hydrogen, alkyl, e.g. C.sub.1 -C.sub.20 alkyl, preferably C.sub.1 -C.sub.4 alkyl, cycloalkyl, e.g. cyclopentyl or cyclohexyl, alkoxy, preferably methoxy or ethoxy, alkylthio with preferably up to 2 carbon atoms, hydroxy. dialkylamino of which the alkyl groups,have preferably up to 2 carbon atoms or halogen, preferably chlorine or bromine; or Y.sup.1 and Y.sup.2 or Y.sup.2 and Y.sup.3 represent the ring members required to complete a fused aromatic ring, preferably a benzene ring, or Y.sup.3 and Y.sup.4 represent the ring members required to complete a fused-on aromatic or cyclohexane ring. Toners within the scope of said general formula are described in GB-P 1,439,478 and U.S. Pat. No. 3,951,660.
Preferably the separated acceptor element is subjected to an overall heat treatment.
In an alternative embodiment the acceptor element contains the radiation to heat converting compound.
Such systems are based on a direct chemical reduction of an organic silver salt, e.g. silver behenate, under the influence of heat. However, due to the pressing together of acceptor and donor, which is normally done under vacuum, an unreproducible inhomogeneous close contact is established between the donor and the acceptor. As a result, after the separation step so-called contact spots tend to appear in the final image due to an irreproducible transfer of donor material. These contact spots give the final image an uneven outlook which is commercially unacceptable. When trying to prevent this defect by incorporating a conventional matting or spacing agent on the surface of donor and/or binder element thus establishing a reproducible more loose contact, as is disclosed in U.S. Pat. Nos. 772,582 and 4,876,235, the obtained density is to low since the chemical reduction is hampered in those local points were spacing particles are present giving rise to a high number of so-called pinholes. The appearance of problems with contact spots and pinholes is not limited to the case where the reacting pair is an organic silver salt and a reducing agent. They will also be present in the case of any pair of a reactant (A) and a reactant (B) that are capable of forming some kind of density by chemical or photochemical reaction with each other.
It is an object of the present invention to provide a method for the formation of a heat mode image which is substantially free of the contact spot defect while providing a sufficiently high density.
It is a further object of the present invention to provide an imaging method, that can serve as an alternative for conventional image-setting based on silver halide films, and that provides an image which can be used for direct visual inspection, e.g. a radiographic image for medical purposes, or as master for the exposure of a printing plate or proofing material.